Thermionic cathodes



Patented June 29, 1954 THERMEONIC CATHODES Leo J. Cronin, Natick, Mass.,assignor to Raytheon Manufacturing Qompany, Newton, Mass, a corporationof Delaware Application December 16, 1950, Serial No. 201,219

Claims.

This invention relates to electron-emitting structures, and moreparticularly to cathodes of the directly-heated therminoic-emissiontype.

The invention discloses an improvement of the directly-heated cathodedisclosed in Patent No. 2,477,601, issued August 2, 1949, to John F.Hanson. This cathode is made from a mixture of powdered refractory metalsuch as molybdenum and thoria, said mixture being compressed into thedesired cathode shape and then heated to an elevated temperature. I

It has been found that cathodes constructed in this manner are extremelyrugged, and are copious electron emitters. However, such cathodes havethe disadvantage that their resistance decreases during the cathodelife. For example, a cathode of this type which has an initial heatervoltage of ten volts for a heater current of eighty amperes will usuallydrop to a heater voltage of six to seven volts for a heater current ofeighty amperes during the processing and operation of the magnetron inwhich the cathode is used.

This invention discloses a means whereby the change in heater voltageduring the processing of the tube may be sharply reduced. Briefly, thisinvention discloses the addition of a small quantity of ceria (ceriumoxide) to the mixture of thoria and refractory metal from which thecathode is made.

Other and further objects and advantages of this invention will beapparent as the description thereof progresses, reference being had tothe accompanying drawing, wherein:

Fig. 1 illustrates a longitudinal, cross-sectional view of a cathode andcathode support embodying this invention; and

Fig. 2 illustrates a transverse, cross-sectional view of the cathode andcathode support structure shown in Fig. 1, taken along line 2-2 of Fig.1.

Referring now to the drawing, there is shown a cathode I 0 comprising ahollow, cylindrical member made of a compressed sintered mixture ofthoria, refractory metal and ceria. The. mixture may be made up, forexample, as follows: approximately sixteen to thirty per cent, byweight, of a refractory metal, metal'compound or metal alloy such asmolybdenum, molybdenum carbide, molybdenum cobalt, molybdenum nickel,tungsten, tungsten carbide, tungsten cobalt or tungsten nickel, pointfive per cent, by Weight, of ceria, and the remainder of thoria. Thismixture is formed into the hollow, cylindrical form of cathode H), forexample,

by means of a mold under pressure which may be, for example, in excessof two hundred thousand pounds per square inch. The compressed,cylindrical member is then sintered by being heated, for example, towithin the temperature range from thirteen hundred to twentyfive hundreddegrees centigrade for from a few minutes to a number of hours dependingupon the characteristics desired.

As shown here, the cathode cylinder I0 is held in a cathode supportcomprising a lower cylindrical member I I having a recess 12 in theupper end thereof adapted to engage the lower end of the cylinder I0. Anupper support member I 3 has a recess I 4 therein adapted to engage theupper end of cylinder [0. A central conductor I5, shown here by way ofexample, extends through cylinders I and I0, and is attached to uppersupport member [3. Thus, it may be seen that cathode cylinder 10 isrigidly held between support members l3 and H in such a manner that, bythe application of a potential between the central conductor l5 and thelower support cylinder I l, a current will flow from central conductorl5 through upper support member l3, cathode cylinder l6, and lowersupport member II. Since members 15, II and I3 are metal, they presentsubstantially no resistance to the current flow, while cathode cylinderl0, being of a mixture of metal, thoria and ceria presents a substantialresistance to current flow, and, as a result, is heated by said currentflow to electron-emitting temperature.

It has been found that a cathode made in accordance with this inventionwill have a relatively small resistance change during the processing andlife of the tube. For example, cathodes of this type requiring aninitial heater voltage of ten point one volts to maintain a current flowof eighty amperes therethrough will retain a heater voltage requirementin excess of nine point eight volts for a current flow of eighty amperesthrough the cathode throughout the processing and life of the tube.

While the precise mechanism of the action of ceria in the cathode is notknown, it is believed that ceria produces a bond that is more stableelectrically, and, as a result, the resistance of the bond remainsrelatively stable throughout the life of the cathode. While, in itspreferred form, the cathode mixture comprises point five per cent, byweight, of ceria, good results may be obtained with mixtures comprisingpoint one per cent to ten per cent, by weight, of ceria.

This completes the description of the particular embodiment of theinvention described here in. However, many modifications thereof will beapparent to persons skilled in the art without departing from the spiritand scope of this invention. For example, many difierent shapes ofcathodes may be used, other well-known electron-emitting materials couldbe substituted for thoria, and the invention is not necessarily limitedto directly-heated thermionic cathodes, since it could also be used withindirectly-heated cathodes. Therefore, it is desired that this inventionbe not limited to the particular details of the species describedherein, except as defined by the appended claims.

What is claimed is: a

1. A directly-heated thermionic cathode comprising a compressed bodyconsisting of thoria, substantially sixteen to thirty per cent, byweight, of a refractory metal, and less than ten -per cent, by weight,of ceria.

ing of thoria, substantially sixteen to thirty per cent, by weight, ofmolybdenum and substantially zero point five per cent, by weight, ofceria.

4. A directly-heated thermionic cathode comprising a compressed bodyconsisting of greater than sixty per cent, by Weight, of thoria, lessthan ten per cent, by Weight, of ceria, and the balance of a refractorymetal.

5. A directly-heated thermionic cathode comprising a compressed bodyconsisting of greater than sixty per cent, by Weight, of thoria,substantially zero point five per cent, by weight, of

ceria, and the valance of a refractory metal.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 568,780 Lowenberg Oct. 6, 1896 1,716,545 Mackay June 11, 19292,473,550 Spencer June 29, 1949 2,501,089 Pomerantz Mar. 21, 1950FOREIGN PATENTS Number Country Date 388,161 France May 23, 1908 3,112Great Britain Feb. 5, 1913

1. A DIRECTLY-HEATED THERMIONIC CATHODE COMPRISING A COMPRESSED BODYCONSISTING OF THORIA, SUBSTANTIALLY SIXTEEN OF THIRTY PER CENT, BYWEIGHT, OF A REFRACTORY METAL AND LESS THAN TEN PER CENT, BY WEIGHT, OFCERIA.